1. Field of Application
The present invention relates to a rotation detection apparatus which detects rotation information concerning a rotor that is formed of a magnetic material, and which utilizes magnetoresistive element (MREs) which change in resistance in response to changes in an applied magnetic field.
2. Description of Prior Art
Types of rotation detection apparatus which utilize MREs are known, for example for obtaining information concerning rotation of a toothed rotor that is provided on the crankshaft or camshaft of an internal combustion engine of a vehicle. Such a type of rotation detection apparatus includes a bias magnet, which produces a bias magnetic field that links with the rotor, and one or more MREs that change in resistance in accordance with changes in a vector of the bias magnetic field. These changes in the magnetic field are processed electrically, to thereby detect changes in the rotation position of the rotor which is the detection object. That is to say, the magnetic vector of the bias magnetic field that links with the detection object (rotor) has periodic angular variations in accordance with the passage of protrusion portions of the rotor periphery and recessed portions of that periphery through the bias magnetic field, as the rotor rotates. The resistances of the MREs vary in accordance with these variations in the magnetic vector of the bias magnetic field, and so vary in accordance with the passing of the protrusion portions and recessed portions of the rotor.
In general with such a rotation detection apparatus, a plurality of MREs are mounted in a sensor chip, and the angular variations of the magnetic vector will differ in accordance with the size of the air gap between the sensor chip and the rotor periphery. That is to say, for the same amount of angular rotation, of the same rotor, the amount of angular variation of the magnetic vector will differ in accordance with the air gap. On the other hand, for the same rotor, if the air gap is altered then it is found that there is a specific “air gap characteristic minimum point”, which is a specific value of magnetic vector angle (corresponding to some specific size of air gap) for which the amounts of increase and decrease of magnetic vector angle with respect to that specific angle (as the rotor rotates) are of equal magnitude. The variations in the magnetic vector angle result in corresponding variations in an output detection signal indicative of changes in rotation angle of the rotor, and the value of output detection signal corresponding to the air gap characteristic minimum point is appropriate for use as a threshold value, for use in converting the output detection signal to a binary signal.
Hence it would be desirable to provide a rotation detection apparatus whereby the air gap characteristic minimum point is held constant, irrespective of changes in the shape of the rotor. If such a type of rotation detection apparatus were available, then operations such as setting of individual detection threshold values in accordance with different shapes of rotor would be substantially reduced.
In that regard, as described for example in Japanese patent publication No. 2003-269995, a type of rotation detection apparatus is known whereby the aforementioned air gap characteristic minimum point is held constant irrespective of rotor shape. With that prior art rotation detection apparatus, a sensor chip is used having an array of four sets of four MREs. Each set of four MREs are connected in series between a power supply voltage and a reference (ground) potential, with a median voltage-divided output being extracted (i.e., whose value would be ½ of the power supply potential, if all four MREs have identical resistance values). Such a median voltage-divided output will be referred to in the following as the median output potential of such a set of MREs. Variations in the median output potential can be used to derive rotation information concerning a rotor.
FIG. 21 shows the configuration of that prior art rotation detection apparatus. As shown, the apparatus includes a sensor chip 104 having a row of four sets of four MREs, the sets respectively designated as A, B, C, D. Each set of four MREs is arranged, as shown, in a square configuration, interconnected such that current first flows successively through a first diagonally opposed pair of MREs, then successively through the second diagonally opposed pair of MREs, with a median output potential being produced with respect to the reference (ground) potential. In the following description and in the appended claims, such a set of four MREs, configured physically and electrically substantially as for each of the sets A, B C, D shown in FIG. 21, will be referred to as a “MRE bridge”, although the electrical configuration is that of a magnetoresistive voltage divider,
Respective median output potentials V1, V2, V3, V4 from these MRE bridges A, B, C, D are inputted to a differential circuit, which performs processing to derive from these a single differential output signal Vd, where Vd=2×(V3−V4)−(V1−V2). By using this differential output signal Vd, the air gap characteristic minimum point can be held constant, irrespective of changes in rotor shape. In the following, “differential output signal” will be abbreviated to “differential output”, for brevity of description.
FIGS. 22, 23 show the output waveforms of the differential output Vd, for different shapes of rotor. It should be understood that the term “changes in rotor shape” as used herein is intended to signify changes in the respective lengths (as measured around the rotor circumference) of protrusion portions and recessed portions of the rotor periphery, i.e., changes in the respective angular extents (as measured with respect to the rotor axis as center) of the protrusion portions and recessed portions. Similarly, the terms “narrow/wide” as applied herein to protrusion portions or to recessed portions respectively signify “relatively short/relatively long in peripheral extent”, i.e., “relatively small/relatively large in angular extent”.
FIG. 22 shows examples of waveforms of the differential output Vd (shown expressed in the form of angular variations of the magnetic vector of the bias magnetic field, as described above) for the case in which both the protrusion portions and the recessed portions of the rotor that is the detection object are relatively narrow. The differential output Vd changes in accordance with the angular variation of the vector of the bias magnetic field, and Vd is expressed in FIG. 22 in terms of the angular variation of that vector. As can be understood from FIG. 22, the air gap characteristic minimum point. (expressed as a value of magnetic vector angle) is approximately 10° with this example.
FIG. 22 shows examples of waveforms of the differential output Vd (again expressed as variations in the magnetic field vector angle) for the case in which both the protrusion portions and the recessed portions of the rotor that is the detection object are relatively wide. Here again, it is found that the air gap characteristic minimum point is approximately 10°.
Thus, by using a set of four MRE bridges in that way, obtaining the differential output Vd from the four output voltages of these MRE bridges, as Vd=2×(V3−V4)−(V1−V2), the air gap characteristic minimum point remains substantially constant, irrespective of changes in the shape of the rotor. Hence, if the value of the air gap characteristic minimum point is used as a threshold value for converting the differential output Vd to a binary signal, rotation information concerning the rotor can be easily and accurately detected.
However as is clear from FIGS. 22, 23, as the shape of the rotor is changed, the waveform of the differential output Vd also changes accordingly. For example with the rotor shape of FIG. 22, the waveform of the differential output Vd changes in a sinusoidal manner in accordance with the passing of the protrusion portions and recessed portions of the rotor periphery. However with the rotor shape of FIG. 23, the waveform of the differential output Vd exhibits abrupt changes between high and low values, due to the phenomenon of magnetic distortion.
FIG. 24 shows examples of waveforms of the differential output Vd (again expressed as variations in the magnetic field vector), for different rotor shapes and different sizes of the air gap. These show the following:
(a) In the case of the rotor shape designated as Sa (having relatively narrow protrusion portions and recessed portions), it can be understood that the maximum and minimum regions of the differential output Vd characteristic respectively correspond to the protrusion portions and recessed portions of the rotor.
(b) In the case of the rotor shape designated as Sb (having relatively narrow protrusion portions and relatively wide recessed portions), it can be understood that the regions of the differential output Vd that correspond to the protrusion portions of the rotor show a relatively sinusoidal variation. However the regions of the differential output Vd that correspond to the recessed portions of the rotor are greatly attenuated at positions corresponding to the centers of these recessed portions.
(c) In the case of the rotor shape designated as Sc (in which both the protrusion portions and the recessed portions are relatively wide) it can be understood that the waveform of the differential output Vd falls abruptly at the respective center positions of the protrusion portions and the recessed portions of the rotor.
For convenience of description, the rotor shapes Sa, Sb, Sc illustrated in FIG. 24 will be referred to in the following as the narrow-protrusion rotor, the equal-pitch rotor and the wide-protrusion rotor, respectively.
Since the waveform of the differential output Vd varies as described above in accordance with the shape of the rotor, this results in slight variations in the degree of latitude (as defined hereinafter) and angular accuracy of detecting rotation information for a rotor based on the differential output Vd, The concepts of “degree of latitude” and “angular accuracy” as used herein will be described referring to FIGS. 25 and 26.
FIG. 25 illustrates the relationship between the waveform of the differential output Vd and various types of air gap, for describing the concept of “degree of latitude”. As shown in FIG. 25, as the air gap becomes larger, the amplitude of the differential output Vd becomes smaller, and reaches a minimum value at positions corresponding to the centers of the protrusion portions and the recessed portions of the rotor.
Considering:
(a) the difference between the minimum value of the differential output Vd that corresponds to a protrusion portion of the rotor (i.e., that is produced while a protrusion portion of the rotor periphery is moving past the MREs) and the air gap characteristic minimum point, and
(b) the difference between the minimum value of differential output Vd that corresponds to a recessed portion of the rotor and the air gap characteristic minimum point;
the smaller of these two values of difference with respect to the air gap characteristic minimum point constituting the “degree of latitude”.
When the degree of latitude falls below a predetermined level, then errors will occur in the detection pulses that are derived by the rotation detection apparatus, so that errors will arise in the rotation information that is derived by the apparatus.
FIG. 26 is a diagram for describing the concept of “angular accuracy” as used herein in describing a rotation detection apparatus. Specifically, FIG. 26 shows a magnified portion of the differential output Vd waveform (i.e., as represented by the angular variation of the bias magnetic field vector) of FIG. 25 (designated as the region S, in FIG. 25). In FIG. 26, the bias magnetic field vector angle corresponding to both the point of intersection PI between the waveforms of the differential output Vd for the case of a “small” air gap and the point of intersection P2 for the case of a “medium” air gap is the air gap characteristic minimum point. There is an angular difference Δα (=α1−α2) between the rotor rotation angles α1 and α2 that respectively correspond to the intersection points P1 and P2, and this angular difference Δα constitutes the “angular accuracy”, as used herein in describing a rotation detection apparatus.
Alternatively stated, the angular accuracy is the degree of accuracy with which rotor rotation angles are attained that respectively correspond to coincidence between the amplitude of the differential output Vd (as the rotor rotates) and the value of Vd corresponding to the air gap characteristic minimum point.
Since the waveform of the differential output Vd varies in accordance with rotor shape, the degree of latitude and angular accuracy in detecting the rotation information also change accordingly. Thus for example when a rotation detection apparatus is used for detecting rotation of the crankshaft or camshaft of the internal combustion engine of a vehicle, the requirements for the degree of latitude and for the angular accuracy will differ, depending upon whether rotation of the crankshaft or rotation of the camshaft is to be detected.
Specifically, in the case of detection of rotation of a camshaft, a rotor (coupled to the camshaft) that is used in conjunction with a rotation detection apparatus will generally be formed with relatively wide peripheral protrusions, for the purpose of accurately discriminating between the respective cylinders of the internal combustion engine. Thus in such an application, a large degree of latitude is more important than a high angular accuracy.
In the case of detection of rotation of a crankshaft on the other hand, a rotor (coupled to the crankshaft) that is used in conjunction with a rotation detection apparatus will generally be formed with relatively narrow peripheral protrusions, for the purpose of accurately detecting the rotation angle of the crankshaft. Thus in such an application, a high angular accuracy is more important than a large degree of latitude.
With a prior art type of rotation detection apparatus, it has not been possible to readily design the apparatus to have optimum rotation detection characteristics for use in such different forms of application.